Sky MPF to Talktalk MPF

I am on Sky MPF (full LLU) and out of curiosity I went through the online order form for Talktalk Essentials. It told me that a new line is required and that I could not keep my existing number. Surprised, I went into live chat and the agent confirmed that migration from Sky MPF to Talktalk is not possible. She said the only way to do it was to move to BT for 18 months then move to Talktalk.

I wonder why this is, since it must be costing Talktalk potential business from current Sky customers. Are Openreach unable to perform MPF to MPF migrations?